The present invention relates to a method and apparatus for providing a body of material with a sub-surface mark using a high energy density beam and additionally relates to a body marked in accordance with the said method or by use of the said apparatus.
Many products are packaged in transparent containers of glass or plastic and there has been a desire for many years to provide containers of this type with a method of marking such that once a mark has been applied, the mark cannot be removed. Such a method of marking would have obvious anti-counterfeiting applications but would also allow for the application of a code specific to each container and so would facilitate product tracking.
It is known for some manufacturers of, for example, more expensive fragrances, to restrict the number and quality of the retail outlets which they authorise to sell their products. As a result, other outlets which desire to retail the same product must do so by utilising illegitimate sources of supply. It is in the manufacturer's interests to curtail any unauthorised transfer of goods which not only can be damaging to the manufacturer's reputation but also can greatly enhance the activities of counterfeiters who are not hindered by the restrictions placed upon registered vendors.
A current system of product tracking employs a method of covertly encoding each container with the identity of the scheduled retailer prior to shipment. However, once the retailer is aware of the presence of the mark, the mark has only to be removed to circumvent the system. If it were possible to provide each container with a truly indelible identification, possibly in the form of a machine readable code such as a bar code, the system would not be so easily overcome and would no longer depend on the covert nature of the mark. Thus, a bar code could be openly applied to the container and, if desired, to its closure, thereby linking the two uniquely. As the filled container progresses to the packing stage, the bar code could be read and copied to subsequent packaging materials by means of a printing, inscribing, engraving or similar process, until both the product and packaging are ready for onward shipment. At this point it would be customary for the shipping destination to be marked on the pack, but if the proposed identification code were machine readable, it could be read at the point at which the identity of the retailer is marked on the pack, and the two correlated by a simple software package. In this way, no matter what is done to the outer packaging, the unique relationship between the product and its intended retailer could still be established from the indelible markings on the container itself.
In the past in order to produce an indelible mark, manufacturers have relied almost exclusively on surface marking. However, the problem with this type of marking is that it may either be destroyed by removing that part of the surface on which the mark is applied or be imitated by the application of an identical mark on a substitute container. In contrast it is an object of the present invention to provide a method of sub-surface marking using a high energy density beam in which the mark is spaced from the surface of the body concerned. Such a mark has the advantage of not only being able to withstand any subsequent surface treatment but also of being very difficult to imitate.
It is known to mark containers using laser radiation but the marks produced often take the form of an engraving or a detectable colour change at a particular surface. For example, U.S. Pat. No. 4,758,703 describes a method of covertly encoding a microscopically visible pattern on a surface of an object in which a beam of unfocused laser radiation is passed through a mask to produce the desired pattern, the intensity of the laser beam being carefully controlled so that the pattern is barely etched onto the surface and remains invisible to the naked eye. U.S. Pat. No. 4,769,310, on the other hand, describes a method of marking ceramic materials, glazes, glass ceramics and glasses that contain at least one radiation-sensitive additive in which a laser beam is focused onto the surface of the material to be marked so as to induce a colour change within the irradiated area.
In contrast, U.S. Pat. No. 3,657,085 describes a method of sub-surface marking using an electron beam but also mentions the possibility of using a laser beam as an alternative. The object of this U.S. patent is to provide a method of marking an article such as a spectacle lens with an identification mark which is normally invisible but which can be rendered visible when required. To this end, the electron or laser beam is focused onto a mask placed over the spectacle lens so that the beam passing through the cut out portions of the mask impinges upon the material of the spectacle lens. The beam is scattered by collisions with the molecules of the material that makes up the lens, with the result that the kinetic energy of the beam is absorbed as heat producing permanent stress patterns within the lens. The stress patterns are invisible to the naked eye but may be rendered visible by double refraction in polarised light.